The treatment of water is currently of constantly increasing importance, in particular, in the field of obtaining drinking water and, also, in the production of high-purity process water. Particularly high demands are made of the water purity, in particular, in the latter case, for example, in the case of process waters which are required in the production of semiconductors. For instance, in semiconductor production, water having an extremely high degree of purity is required for washing silicon wafers, in particular, after etching processes.
The starting point for producing the high-purity water required is frequently surface water, for example, river water. This is purified in a multistage process comprising a pretreatment section, what is termed a “make-up” section, and what is termed a “polishing” section. The pretreatment generally comprises, in particular, one or more filtration operations for removing fine particles, a flocculation step for removing colloid substances and very fine, dirt particles, a sterilization, a softening and a desalting of the raw water. Subsequently, the water thus treated is, in the make-up section, degassed, deionized and treated with UV. A further UV treatment can be provided during polishing. In addition, the polishing generally comprises further deionization processes and also at least one ultrafiltration step.
The raw water to be treated generally contains organic constituents or impurities which must be removed as far as possible during the treatment. Occasionally, the total content of organic constituents in the raw water can even exceed a value of 5 ppm (parts per million).
For water which is intended to be used in semiconductor production, generally a maximum value of approximately 1 ppb (part per billion) of organically bound carbon, total organic carbon (TOC) is sought. Particular attention is paid here to removing nitrogenous organic compounds, in particular, urea and urea derivatives, since these have proved to be particularly interfering in semiconductor production. However, experience has shown especially that low-molecular-weight nitrogenous organic compounds may be removed only with great difficulty in conventional water treatment processes.
Surface water frequently has a very high concentration of nitrogenous organic compounds, in particular in regions utilized intensively for agriculture. In these regions the concentration of nitrogenous organic compounds in surface water is increased primarily by the intensive use of nitrogen-based fertilizer.
Rydzewski et al. (Ultrapure Water® November 2003, pages 20-26) propose adding sodium bromide and ozone to a water stream for removing nitrogenous organic compounds, in particular urea. By means of the ozone a part of the added sodium bromide is oxidized, and hypobromite is formed which in turn can react with nitrogenous organic compounds and so convert these into a state in which they can be more easily removed from the water stream. The ozone and the sodium bromide are added separately for this to the water stream, wherein according to Rydzewski et al. the sodium bromide must be added upstream of the ozone addition to achieve optimum mixing of the two components in the water stream. Rydzewski et al. found that in this manner the fraction of nitrogenous organic compounds in a water stream may in fact be effectively reduced. However, in their studies they came to the result that an efficient reduction of this fraction can only be achieved by an extraordinarily high hypobromite concentration (for a breakdown of urea from 25 ppb to 5 ppb, a hypobromite concentration of approximately 20 ppm is recommended). Correspondingly they propose adding sodium bromide and ozone to the water stream in very high amounts which of course disadvantageously greatly increases the total ionic load of the water stream. Downstream separation stages are especially occasionally considerably impaired by the ionic loading which is introduced. Furthermore, high concentrations of the strong oxidizing agent hypobromite can cause considerable material problems. In particular in the case of ozone this is moreover a relatively expensive reagent, and it must be freshly generated immediately before use with a high demand of equipment resources. Also from economic aspects, the procedure proposed by Rydzewski et al. is therefore capable of being optimized.
Separating off nitrogenous organic compounds from a water stream by means of hypobromite is also a topic of JP 09094585. From this it is known to add sodium bromide and sodium hypochlorite in combination with a flocculent to a water stream which is to be treated. Sodium hypochlorite, just as is ozone, is able to oxidize the bromide to hypobromite. In a first step the flocculent and the sodium bromide are added to the water stream. Not until after the substances which have flocculated out have been separated off by means of a filter is the sodium hypochlorite added separately to the water stream in a further step. However, this procedure already does not appear to be optimal, because a flocculation step is generally performed in a very early stage of a water treatment process, namely in the context of the pretreatment (see above). In this early stage the water stream which is to be treated still has high contamination due to impurities of all types. A targeted reaction of nitrogenous organic impurities with hypobromite is therefore impossible, which means that hypobromite must be generated in very high concentrations to ensure efficient removal thereof.
It could therefore be helpful to provide a technical solution for treating water, in particular, for producing ultrapure water in which especially the aspect of the targeted breakdown of nitrogenous organic compounds is taken into account.